EP0646544B1 - Process and apparatus for the preparation of fullerenes - Google Patents

Abstract

A process for preparing cage molecules known as fullerenes comprising carbon atoms having closed networks, in particular of pentagonal and/or hexagonal rings, in a reaction vessel facility is characterised by the process steps: introduction of a gas mixture containing between 10% and 100% of carbon monoxide (CO), in particular from 30% to 60%, and generation of a glow discharge within the reaction vessel facility. Using this process, the yield for preparing cage molecules can be optimised and the composition can be varied. An apparatus for preparing the cage molecules is characterised by a reaction vessel facility (1) within which a glow discharge, in particular a direct current glow discharge (DC glow discharge), can be generated, and at least one electrode (4,5) which consists at least in part of a graphite body which can be heated to white heat by means of an additional current. <IMAGE>

Description

TECHNICAL AREA

. Darin bedeuten m die Anzahl der Sechserringe (Hexagone). Es hat sich gezeigt, daß die Fullerene mit der Kohlenstoffatomzahl X = 60 und größer besonders stabil sind. Die Erfindung betrifft weiterhin eine Vorrichtung zur Herstellung von Fullerenen mit einer Reaktionsbehältereinrichtung und Elektroden.The invention relates to a method for producing fullerenes. Fullerenes are hollow or cage molecules whose shells consist of a closed network of pentagonal and hexagonal rings made of carbon atoms. The number of carbon atoms X results from the equation $\text{X = 20 + 2 * m}$

. Here m means the number of rings of six (hexagons). It has been shown that the fullerenes with the carbon atom number X = 60 and larger are particularly stable. The invention further relates to a device for producing fullerenes with a reaction vessel device and electrodes.

STATE OF THE ART

For the first time, Krätschmer and others were able to produce fullerenes in weighable quantities by evaporating an electrically heated graphite rod in a helium atmosphere, as described in Nature, Vol 347 (1990), p. 354 ff. The fullerenes can be washed out of the soot, which is deposited in the test vessel, by organic solvents such as toluene or benzene.

After the first publication by Krätschmer, a number of improvements and alternative processes by which fullerenes can be produced have become known. For example, in the book "Buckminsterfullerenes", VCH Verlagsgesellschaft mbH, Weinheim (1993), p. 302 ff, a reactor vessel is described which allows the fullerenes to be produced more easily by the Krätschmer process. Another method, published in the journal Spektrum der Wissenschaft, December 1991, 88 ff is based on the fact that carbon evaporates from the graphite electrodes of an arc and is deposited on the walls of the arc vessel as soot, which contains fullerenes as a component. WO 93 01 128 A1 describes a method in which a carbon rod is made electrically conductive by heating and then at least partially evaporated by electrical heating in an oxygen-free atmosphere. In contrast, European patent application EP 0 536 500 A1 describes a process in which fullerene production takes place in an oxygen-containing atmosphere. European patent application EP 527 035 describes a process for the production of fullerenes in which carbon-containing particles of small diameter are placed in an arc and the particles evaporate due to the energy of the arc, and when the steam cools outside the hot core of the arc fullerene form. WO 92 20 622 A1 describes a process for the production of fullerenes by burning carbon-containing compounds under compliance with special conditions. In all of these processes, carbon is first brought to a high temperature of more than 2000 ^o C; when the carbon diffuses from the hot area into colder zones of the reaction vessel, the carbon atoms are combined to form the fullerenes under conditions which have not yet been fully understood. This diffusion process can only be controlled inadequately by the pressure and type of the buffer gas, temperature or current intensity of the arc or the power of the electrical heating of the graphite rod. As a result, the yields in the production of the fullerenes are only low and the fullerenes with the carbon numbers 60 and 70 (C ₆₀ and C ₇₀ ) are mainly formed. Fullerenes with higher carbon numbers have only been detected in negligible amounts.

PRESENTATION OF THE INVENTION

The invention is based on the technical problem or the object of providing a process which is improved compared to the prior art mentioned and which enables a simple and economical apparatus construction with which a high yield of cage molecules can be achieved with which the conditions are selectively and sensitively adjustable for the formation of the cage molecules. In addition, it is an object of the invention to provide a device for performing the above. Procedure. The inventive method is given by the features of claim 1. The inventive device for manufacturing is given by the features of claim 9. Advantageous refinements and developments are the subject of the dependent claims.

The process according to the invention is characterized by the following process steps: introducing a gas mixture which contains between 10% and 100% carbon monoxide (CO), in particular 30% to 60%, and igniting a glow discharge within the reaction vessel device.

The formation of the fullerenes takes place in a glow discharge, which preferably fills the reaction vessel largely uniformly. In particular, use is made of the fact that in a glow discharge the gas temperature can be much lower than the electron temperature given by the mean kinetic energy of the electrons. For example, the electron temperature in glow discharges, which are used for the excitation of CO ₂ gas lasers, is between 1.0 eV and 1.5 eV (corresponding to approximately 11,000 K and 16,500 K), while the gas temperature is below 200 ^° C. For the synthesis of the fullerenes, the gas temperature is now set in the glow discharge so that the fullerenes are stable at this temperature and do not decay. The energy required for the fusion of the carbon atoms the ordered structures of the fullerenes are added to the chain or ring-shaped carbon clusters, which are the precursors of the fullerenes, by collisions with the electrons. In addition, energy transfer to the carbon clusters can take place through inelastic collisions with excited gas particles.

$${\text{CO + e → C}}^{\text{+}}\text{+ O + 2e}$$

$${\text{CO + e → C + O}}^{\text{+}}\text{+ 2e}$$

For the production of the fullerenes according to the invention, use is made of the known fact that carbon in the form of soot precipitates from glow discharges with a high proportion of carbon monoxide (CO). (Gmelin, Handbook of Inorganic Chemistry, Carbon Part C2, page 6, Verlag Chemie, Weinheim (1972)). The reason for this is the dissociative ionization of the CO molecules through electron collisions. According to Gmelin, Handbook of Inorganic Chemistry Part C, Delivery 1, p. 127 ff, Verlag Chemie, Weinheim (1970), the essential processes in pure CO gas are: $${\text{CO + e → C}}^{\text{+}}{\text{+ O}}^{\text{-}}\text{+ e}$$

$${\text{CO + e → C}}^{\text{+}}\text{+ O + 2e}$$

$${\text{CO + e → C + O}}^{\text{+}}\text{+ 2e}$$

With gas mixtures with noble gases or other molecular gases, the number of reactions taking place increases rapidly. In particular, with a longer dwell time of a gas volume in the glow discharge, secondary products arise which shift the equilibrium. In addition to carbon and oxygen, CO also forms from CO ₂ .

In addition to the parameter range specified by Gmelin, the glow discharge can also be operated at higher pressures up to a few hundred mbar if the discharge is excited by a capacitively coupled high frequency. Such arrangements are used, among other things, to excite gas lasers and are described in EP 0 275 023. Also DC discharges in tubes with several centimeters Diameters up to pressures of 50 mbar and more can be operated as a glow discharge if it is ensured that the gas temperature does not exceed a value of about 350 ^° C. At higher temperatures there is a risk that an arc will occur which can lead to the destruction of the discharge vessel. To check the gas temperature, either the wall of the discharge vessel must be cooled or the gas must be replaced so quickly that the power loss of the glow discharge is dissipated with the gas stream sufficiently quickly.

A preferred embodiment is characterized in that the electrical energy supply is clocked, the pulse duty factor can be between 10% and 90%. The energy density can thus be controlled by supplying electrical energy from 10% to 90% of a clock period or not.

• Gaszusammensetzung,
• Druck,
• elektrische Leistungsdichte,
• Gastemperatur,
• Verweildauer des Gases im Glimmentladungsraum, d. h. die Strömungsgeschwindigkeit des Gases.

For the production of fullerenes in a glow discharge, the following sizes are selected and matched in particular:

• Gas composition,
• Pressure,
• electrical power density,
• Gas temperature,
• Residence time of the gas in the glow discharge space, ie the flow rate of the gas.

The use of helium in addition to carbon monoxide is advantageous because the first excited level with helium is metastable at approx. 20 eV, i. that is, the helium can give relatively large amounts of energy to carbon clusters during inelastic collisions. Good results can be achieved with a mixture of 40% to 60% CO and 60% to 40% helium.

The pressure in the glow discharge should be as high as possible, so that the carbon density that occurs in the dissociative Ionization of the CO arises, is as large as possible. On the other hand, the glow discharge tends to form arcs with increasing pressure. With a high-frequency excitation, glow discharges can be operated easily mbar up to 150, as long as the gas temperature does not increase above about 300 ^° C.

Power density, pressure and gas composition cannot be set completely independently of one another; rather, a state arises in the stationary gas discharge in which the generation rate and destruction rate of the charge carriers are the same. This is generally the case for a given pressure and gas composition only in a limited range of power density. For the above conditions, this is between 50 and 130 watts / cm ³ .

For the stationary case, the gas temperature results from the coupled electrical power and the cooling conditions of the gas discharge. The power loss can be dissipated via the walls of the discharge vessel, which are cooled, for example, by water, or by continuously exchanging gas and dissipating the power loss as sensible heat of the gas mixture. A combination of both methods is also easy to implement.

. Ein weiterer Vorteil liegt darin, daß die Fullerene, die sich in der Glimmentladung bilden, mit der Strömung aus dem Reaktionsraum getragen werden und beispielsweise durch eine Waschflasche mit einem organischen Lösungsmittel wie Benzol oder Toluol ausgewaschen werden können.In addition to the removal of heat, the continuous gas exchange has the further advantage that the gas composition changes only insignificantly during the limited residence time of the gas in the glow discharge space. The dwell time T is obtained from the flow velocity v and the length of the discharge chamber L. $\text{T = L / v}$

. Another advantage is that the fullerenes that form in the glow discharge are carried with the flow out of the reaction space and can be washed out, for example, with a wash bottle with an organic solvent such as benzene or toluene.

Since the gas pressure cannot be chosen to be as high as would be desirable for the formation of the fullerenes for reasons of the stability of the glow discharge, the density of the carbon atoms and carbon clusters of low atomic numbers in the glow discharge space can be increased by the vapor of a hot graphite body being introduced into the glow discharge vessel is directed. This can be done either by diffusion into the glow discharge space or by loading the inflowing gas with the steam.

The device according to the invention for the production of cage molecules has a reaction container device within which a glow discharge, in particular direct current glow discharge, and at least one electrode, and is characterized in that the electrode consists at least in regions of a graphite body which can be heated to white heat by an additional current .

Further embodiments and advantages of the invention result from the features further specified in the claims and from the exemplary embodiments specified below. The features of the claims can be combined with one another in any way insofar as they are not obviously mutually exclusive.

BRIEF DESCRIPTION OF THE DRAWING

Fig. 1

schematische Darstellung einer Reaktionsbehältereinrichtung für eine Gleichstromglimmentladung zur Herstellung von Fullerenen,

Fig. 2

schematische Darstellung einer Reaktionsbehältereinrichtung für eine hochfrequenzangeregte Glimmentladung zur Herstellung von Fullerenen,

Fig. 3

Querschnitt eines Entladungsgefäßes für eine Hochfrequenzglimmentladung gemäß Fig. 2,

Fig. 4

schematische Detaildarstellung einer Gleichstromreaktionsbehältereinrichtung mit einem heizbaren Graphitkörper und

Fig. 5

schematische Detaildarstellung einer Hochfrequenzreaktionsbehältereinrichtung mit einem heizbaren Graphitkörper.

The invention and advantageous embodiments and developments thereof are described and explained in more detail below with reference to the examples shown in the drawing. The features to be gathered from the description and the drawing can be used according to the invention individually or in groups in any combination. Show it:

Fig. 1

schematic representation of a reaction vessel device for a direct current glow discharge for the production of fullerenes,

Fig. 2

schematic representation of a reaction vessel device for a high-frequency excited glow discharge for the production of fullerenes,

Fig. 3

Cross section of a discharge vessel for a high-frequency glow discharge according to FIG. 2,

Fig. 4

schematic detail representation of a DC reaction vessel device with a heatable graphite body and

Fig. 5

schematic detailed representation of a high-frequency reaction container device with a heatable graphite body.

WAYS OF CARRYING OUT THE INVENTION

In FIG. 1, 1 denotes a discharge vessel (reaction vessel device) for direct current glow discharge, which encloses a discharge space 2, in which the gas mixture supplied through a gas line 8 is excited. The discharge vessel 1 is double-walled, so that cooling water can be passed through a cylindrical cavity 3, 3 '. The connections required for this are not shown here. The electrical excitation takes place via a high-voltage device 6, which is connected to electrodes 4 and 5 via the leads 15 and 16. The electrodes 4 and 5 protrude into the discharge space 2. A series resistor 7 is used to limit the current through the discharge. The gas mixture is fed into a closed circuit by means of a pump 12 via gas lines 8, 9, 11 and 13 through the discharge space 2 and through a wash bottle 10 filled with an organic solvent.

The flow can be adjusted with a valve 14. To a small extent, fresh gas is advantageously supplied continuously and part of the circulating gas is exchanged (not shown in FIG. 1).

2 shows an arrangement according to the invention, in which, in contrast to FIG. 1, the gas discharge is operated via a high-frequency generator. The gas cycle is identical to that from Fi. 1. The same components have the same reference numerals as in Fig. 1 and are not explained again. A discharge vessel 21 encloses a gas space 22, in which a high-frequency discharge is generated by means of electrodes 24 and 25, which extend over the entire length of the discharge vessel 21. With 26 a high-frequency generator is designated, which also contains the necessary electrical matching elements. The reaction vessel device described can be operated in the same way with low-frequency alternating current, preferably between 50 Hz and 400 Hz. For this purpose, only the high-voltage device 6 has to be suitably selected.

FIG. 3 shows a cross section of the high-frequency discharge vessel according to FIG. 2. The discharge vessel 21 itself consists of an electrically non-conductive ceramic material, for example Al ₂ O ₃ . Cooling bores 23 and 23 'through which cooling water can be pumped are embedded in the upper and lower walls. The electrodes 24 and 25 consist of a metallic material such as aluminum, copper or stainless steel. It is also possible to use the catalytic effect of metallic surfaces in order to advantageously influence the composition of the gas mixture, which changes as a result of the discharge.

In FIG. 4, a DC discharge vessel with a discharge space 42 is schematically designated by 41. An electrode 43 is designed as a graphite body, which by an alternating current can be heated to white heat. 45 is the supply line for the current through the gas discharge. The alternating current is generated by means of an isolating transformer 44 and fed to the electrode 43 via feed lines 46, 47.

A high-frequency discharge vessel with a discharge space 52 is shown schematically in FIG. 5 at 51. A small part of a high-frequency electrode 54 is replaced by an elongated graphite body 53 which can be heated to white heat by an alternating current. The alternating current of suitable current strength and voltage is generated via an isolating transformer 55 and fed to the graphite body via corresponding supply lines 56 and 57.

The invention is explained in more detail using the example of a glow discharge excited with high frequency. A 60 cm long discharge vessel made of Al ₂ O ₃ , which is cooled with water according to FIG. 2 and has internal electrodes made of anodized aluminum, is connected on one side to a gas bottle with premixed gas made of 50% helium and 50% CO. On the opposite side, a gas line leads to a wash bottle with toluene. A pump is connected to the outlet side of the wash bottle. In contrast to Fig. 2, the gas is not circulated. The discharge vessel is connected to a high-frequency generator with a frequency of 56 MHz, the maximum power is 1000 W. The inflow of the gas and the pump power are set so that the gas remains in the discharge space for about 5 seconds. Raw fullerene, which mainly contains the atomic numbers 60 and 70, accumulates in the wash bottle over an operating period of 10 minutes.

By changing the operating parameters such as gas composition, pressure, power density and flow rate, the yield can be optimized and the composition of the raw filler can be varied. The loading of the Gas flow with graphite vapor in front of the discharge vessel or on the inflow side in the discharge vessel.

Another advantage of the invention is that fullerenes are formed in the volume of the discharge. This makes it possible to produce fullerenes on a larger scale in large-volume glow discharges. Similar glow discharges are used, for example, in high-performance CO ₂ lasers for material processing. The process is therefore scalable up to plants for the commercial production of fullerenes.

Claims (15)

1. Method and apparatus for production of cage molecules of carbon atoms with closed networks, especially of pentagonal and hexagonal rings, called fullerenes, in a reaction chamber device,
   characterized by

   - feeding of a gas mixture, which contains between 10% and 95% carbon monoxide (CO), especially 30% to 60%, and

   - exciting a glow discharge within the reaction chamber device.
2. Method according to claim 1,
   characterized in that

   - the glow discharge occupies in essence homogenuosly the reaction chamber device.
3. Method according to claim 1 or 2,
   characterized in that

   - a gas mixture is feeded in, which contains in addition to CO also helium (He) and/or argon (Ar) and/or carbon dioxide (CO₂).
4. Method according to any one or several of the preceding claims,
   characterized in that

   - the glow discharge is excited by direct current (dc).
5. Method according to any one or several of the preceding claims,
   characterized in that

   - the glow discharge is excited by alternating current of low frequency, especially between 50 Hz and 400 Hz.
6. Method according to any one or several of the preceding claims,
   characterized in that

   - the glow discharge is excited by high frequency, whereby the frequency can be between 100 kHz and 10 GHz.
7. Method according to any one or several of the preceding claims,
   characterized in that

   - the energy transmission is supplied with clock pulses, whereby the pulse-duty factor can be between 10% and 90%.
8. Method according to any one or several of the preceding claims,
   characterized in that

   - in addition carbon vapor of a hot graphite body can diffuse into the volume of the glow discharge.
9. Method according to any one or several of the preceding claims,
   characterized in that

   - the gas mixture is flowing through the glow discharge chamber and after leaving the glow discharge volume the fullerenes are gained by washing out with an organic solvent.
10. Method according to claim 9,
    characterized in that

    - the gas mixture is circulated and optionally added fresh gas mixture.
11. Apparatus for production of cage molecules, especially according to any one or several of the method claims 1 to 10, with
    a reaction chamber device, in which a glow discharge, especially a direct current glow discharge, is exciteable, and
    at least one electrode,
    characterized in that

    - the electrode consists at least in parts of a graphite body, which is heatable up to white heat by an additional current.
12. Apparatus according to claim 11,
    characterized by

    - a high frequency device, which works especially in a frequency range from 100 kHz to 10 GHz and which excites the glow discharge by capacitiv coupling.
13. Apparatus according to claim 11 or 12,
    characterized in that

    - an inflow and/or outflow opening for a gas mixture is apparent.
14. Apparatus according to claim 13
    characterized in that

    - the electrode with the graphite body is arranged near to the inflow opening.
15. Apparatus according to any one or several of the preceding claims 11 to 14,
    characterized in that

    - the walls of the reaction chamber device can be cooled at least in parts.

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